1. Technical Field
The invention relates to a process for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation and more particularly to a process for enhancing the recovery of hydrocarbons from a subterranean hydrocarbon-bearing formation by flooding the formation with fluids.
2. Background Information
It has been speculated that flooding of a subterranean oilbearing sandstone formation with alternating slugs of water and a gas can improve oil recovery from the formation over conventional secondary recovery means, such as waterflooding. See, for example, Pfister, R. J., "More Oil From Spent Water Drives By Intermittent Air or Gas Injection", Producer's Monthly, pp. 10-12, September, 1947, which suggests that water-alternating-gas (WAG) flooding is superior to conventional waterflooding in the sandstone Bradford Field of western Pennsylvania. U.S. Pat. No. 1,658,305 to Russell suggests an oil recovery mechanism for WAG flooding in sandstone formations.
Subsequent to these references, a number of modifications and improvements to the basic WAG process have developed in the art as exemplified by U.S. Pat. Nos. 3,244,228 to Parrish, 3,525,395 and 3,525,396 to Chew, and 3,882,940 to Carlin as well as Champion, J. H., et al, "An Immiscible WAG Injection Project in the Kuparuk River Unit", Society of Petroleum Engineers Paper No. SPE 16719, presented in September 1987. All of these references demonstrate the utility of WAG flooding in homogeneous sandstone formations.
References also exist which disclose the utility of cyclically flooding heterogeneous formations with alternate fluids. Gorbanetz, V. K., et al, "Effect of Layered Inhomogeneity of the Formation on Oil Displacement by Enriched Gas", Neftyanoe Khozyaistvoe, n. 8, 1975, pp. 36-37, WAG floods a heterogeneous formation with an enriched gas under miscible conditions. The heterogeneous formation of Gorbanety et al contains two or more isolated homogeneous oilbearing strata of differing permeabilities.
U. S. Pat. No. 3,493,049 to Matthews et al cyclically floods a heterogeneous formation with water, gas, and an oxidizing agent. The heterogeneous formation of Matthews et al contains fractures, channels, lenses or networks of differing permeability or porosity. Matthews et al is not a true WAG flooding process because in practice it requires pressure pulsing and the injection of a separate oxidizing agent slug in addition to the water and gas slugs.
It is apparent that the art generally recognizes the utility of WAG flooding processes in certain types of formations. However, a number of formations exist other than those described above in which WAG flooding processes are not believed to improve oil recovery. For example, WAG flooding is not believed to be effective in formations where the producing stratum or zone contains a residual light crude oil and comprises two or more rock types of differing permeabilities. Thus, a need exists for a process to effectively recover oil from formations exhibiting these characteristics.